SAN FRANCISCO — A Chinese supercomputer has been ranked as the world’s second-fastest machine, surpassing European and Japanese systems and underscoring China’s aggressive commitment to science and technology.

The Dawning Nebulae, based at the National Supercomputing Center in Shenzhen, China, has achieved a sustained computing speed of 1.27 petaflops — the equivalent of one thousand trillion mathematical operations a second — in the latest semiannual ranking of the world’s fastest 500 computers.

The newest ranking was made public Monday at the International Supercomputer Conference in Hamburg, Germany. Supercomputers are used for scientific and engineering problems ranging from climate simulation to automotive design.

The Chinese machine is actually now ranked as the world’s fastest in terms of theoretical peak performance, but that is considered a less significant measure than the computing speed achieved on a standardized computing test.

The world’s fastest computer remains the Cray Jaguar supercomputer, based at the Oak Ridge National Laboratory in Tennessee. In November it was measured at 1.75 petaflops.

In the previous year’s ranking, the Chinese had the fifth-fastest computer, a system that was based at a National Supercomputing Center in Tianjin, China. That machine has dropped to seventh place.

The United States continues to be the dominant maker of supercomputers, and is the nation with the most machines in the top 500. The United States has 282 of the world’s fastest 500 computers on the new list, an increase from 277 when the rankings were compiled in November.

But China appears intent on challenging American dominance. There had been some expectation that China would make an effort to complete a system based on Chinese-designed components in time for the June ranking. The Nebulae is based on chips from Intel and Nvidia.

The new system, which is based on a microprocessor that has been designed and manufactured in China, is now expected later this year. A number of supercomputing industry scientists and engineers said that it was possible that the new machine would claim the title of world’s fastest.

“I wouldn’t be surprised if by the end of this year they surpass the scientific computing power of the EU countries combined and have a computer system with an achieved performance to reach the No. 1 position on the top 500,” said Jack Dongarra, a computer scientist at the University of Tennessee and one of the researchers who has organized the twice-yearly rankings.

Americans designed the first machines that were defined as supercomputers during the 1960s, and the United States has rarely been dislodged from its controlling position as technology leader. In 2002, however, the Japanese Government’s Earth Simulator set off anxiety in Washington when that system briefly claimed the position.

The United States then began investing heavily in the computing systems, breaking the petaflop barrier in 2008.

It is now preparing to begin a sustained push to build systems capable of computing at what is known as exascale performance — 1,000 times faster than today’s fastest systems.